The present invention relates to a hybrid car for driving either of front and rear wheels by an engine and driving the other wheels by a motor.
Patent Document 1 (Japanese Application Patent Laid-open Publication No. Hei 07-231508) describes a four-wheel driving device for converting power to electrical energy using an engine as a power source, driving a motor by the electrical energy, thereby driving either of front and rear wheels.
Patent Document 2 (Japanese Application Patent Laid-open Publication No. 2003-301725) discloses an invention for operating an exhaust turbo-rotary machine as a motor by generated power of a rotary machine connected to an engine power shaft, assisting turbo-supercharging, thereby increasing torque of the engine.
The driving device for a hybrid vehicle disclosed in Patent Document 1 is said to be easily mountable due to a small battery. Such a device is considered to be suitable especially for a small car having a limited space.
On the other hand, a small car generally loads a small engine, and the output of an engine, which can be used for power generation, is limited, so that motor output for sufficiently driving wheels can be hardly obtained, and there is the possibility that the acceleration and running property on a bad road such as a frozen road or a muddy road, which is required by four-wheel drive, cannot be obtained sufficiently.
Moreover, particularly when supplying power of a generator directly to the motor, the efficiency of converting the mechanical energy from the engine to electrical energy by the generator and furthermore the efficiency of converting the electrical energy to the mechanical energy by the motor are concerned with it, so that a problem arises that the efficiency as a whole is reduced and the necessary output of the motor can be hardly obtained.
Therefore, it may be considered to temporarily supply power to the motor by using a battery. However, the stored electricity quantity of the battery is limited and there is the possibility that the acceleration and running property on a bad road cannot be maintained for many hours.
Further, since the efficiency is low, when driving a wheel driving motor in a road surface state requiring no four-wheel drive, there is the possibility of increasing fuel consumption.
On the other hand, as disclosed in Patent Document 2, when driving a supercharger by the motor and increasing the torque of the engine, the torque for rotating the wheels is increased.
At this time, in a satisfactory road surface state, the starting acceleration property is improved, while on a bad road, a slip of the wheels is caused inversely and the vehicle enters a state hard to drive.
Further, although the engine torque is increased, fuel is consumed in correspondence to it, so that a problem arises that fuel consumption is increased.